1. Field of the Invention
The present invention relates to an apparatus and method for measuring a velocity of a moving object in a navigation system, and more particularly to an apparatus and method for measuring a velocity of a moving object in a navigation system that can increase an accuracy of a velocity measurement and a position estimation of the moving object.
2. Description of the Related Art
A position-measuring device for obtaining the position of a moving object is installed in a navigation system of the moving object. Such position measuring devices are classified into one of two types: one type is for determining die position of a moving object using external assistance, and the other is for determining the position of a moving object using an internal system. The external assistance may be in the form of a Global Positioning System (GPS), and the internal system may be in the form of a Dead Reckoning (DR) system. The DR system is a system that calculates a velocity and position of the moving object using data measured by a sensor, such as an accelerometer, a gyroscope (hereinafter referred to as a “gyro”), etc.
FIG. 1 is a block diagram schematically illustrating a conventional apparatus for measuring a velocity of a moving object in a navigation system.
Referring to FIG. 1, the conventional velocity measuring apparatus includes a sensor part 10 and a calculation part 20. The sensor part 10 includes an accelerometer 14 for sensing a traveling acceleration and a traveling direction of a moving object. The sensor part 10 further includes a first gyro 12, and includes a second gyro 16 for sensing an inclination angle of an inclined surface on a traveling path of the moving object. The calculation part 20 includes a velocity calculation unit 22 for calculating a velocity of the moving object and a gravity component calculation unit 24.
The velocity of the moving object can be determined by combining velocity information obtained by the accelerometer 14 with direction information obtained by the first gyro 12. In order for a DR system, which includes an inertia sensor, to determine a traveling velocity vector of the moving object, the DR system is required to measure a traveling direction angle of the moving object and the velocity of the moving object in a traveling direction. In order to determine the traveling direction angle, the first gyro 12, which is installed on an axis that is perpendicular to a plane including an axis of rotation of an axle of the moving object, is required. Also, in order to determine the velocity in the traveling direction, in addition to the accelerometer 14 installed in an axle direction, the second gyro 16, which measures a road inclination angle corresponding to an angle of the plane placed on the axle, is required.
The road inclination angle is defined as an inclination angle against a plane that is perpendicular to a direction of gravity acceleration. The DR system removes a gravity acceleration component included in a measured value of the accelerometer 14 by measuring the road inclination angle.
If an axis direction of the accelerometer 14 installed in the moving object is not perpendicular to the direction of the gravity acceleration on a traveling path of the moving object, the value measured by the accelerometer 14 includes a part of the gravity acceleration component. The inclusion of such a gravity acceleration component causes a large error in measuring the velocity of the moving object. Accordingly, the DR system is provided with the second gyro 16, or an inclinometer, in order to remove the gravity component.
The direction of the gravity acceleration is always towards the center of the earth and perpendicular to the surface of the earth ellipsoid (i.e. spherical surface approximating the surface of the earth). In the DR system, when the axis direction of an accelerometer 14 mounted on a moving object (e.g., fixed, in a specified direction, to a car body) is changed due to the change of the road inclination angle, the DR system must remove the gravity component appearing at an output of the accelerometer 14 in order to determine a pure movement acceleration of the moving object. However, if information on the road inclination angle is not provided, it is impossible to discriminate between the movement acceleration in the traveling direction and the gravity acceleration component, and thus the pure velocity of the moving object cannot be determined.
As described above, in order to obtain the pure movement acceleration of the moving object in the DR system, a sensor for measuring the road inclination angle is required. In the conventional DR system, the second gyro 16 mainly serves as such a sensor. Since the gyros 12 and 16 are sensors for measuring a variation rate of a rotation angle, the road inclination angle can be obtained by integrating the output of the second gyro 16. The second gyro 16 has an advantage in that the second gyro 16 is suitable for tracking an instantaneous change of the inclination angle. However, an error component of the second gyro 16 is integrated in the process of obtaining the inclination angle, and thus an estimated error of the inclination angle measured by the second gyro 16 accumulates over time.
Accordingly, since the second gyro 16 has the disadvantage of error component accumulation, there is a need for a method of accurately measuring the inclination angle of a moving object without using the second gyro 16.